Summary Alzheimer?s disease (AD) is the most common cause of dementia and affects approximately 50 million people worldwide. Although significant resources have been invested, successful therapies have yet to be discovered, suggesting that alternative approaches may be needed. This proposal will investigate a new idea connecting modifications spontaneously occurring in long-lived proteins to the underlying causes of AD. These modifications, known as isomerization and epimerization, represent structural changes that significantly perturb the behavior of affected proteins. These modifications have largely escaped prior investigation because they have historically been difficult to detect, but recent advances in technology have enabled their identification and characterization on a larger scale than previously possible. We will use these methods to fully characterize these modifications in a protein called tau and the amyloid beta peptide, which are both intricately involved in the progression of AD. We will also explore the underlying chemistry leading to the formation of isomerization and epimerization and the rates at which they accrue. Importantly, we hypothesize that these modifications prevent long-lived proteins from being broken down and recycled in the lysosome. This hypothesis is supported by preliminary results and, importantly, provides a pathway that could eventually explain the lysosomal malfunction that is known to occur in AD and is among the earliest observable problems at the cellular level. We will perform experiments to determine whether any of the lysosomal proteases, the molecules responsible for degrading proteins, are able to digest the modified proteins. Undigested protein fragments persisting in the lysosome due to isomerization or epimerization would be subject to accumulation. This failure closely parallels events occurring with undigested substrates in lysosomal storage disorders and provides a potential explanation for the previously observed connections between the two diseases, which exhibit similar pathology. The ultimate goal of this project is to establish that modifications to long-lived proteins initiate events that ultimately lead to lysosomal failure in AD, which will open up new therapeutic strategies for exploration.